Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask that is placed between the light source and the plate.
The exposed plate is usually developed with a liquid developer such as an aqueous alkaline developer to bare the substrate in the non-hardened or solubilized areas. On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, 6,482,571, 6,737,220, 6,994,028, 6,969,575, and 6,949,327.
Laser sources have been increasingly used to imagewise expose a lithographic printing plate that is sensitized to a corresponding laser. This allows the elimination of the photomask film, reducing material, equipment and labor cost. Among the lasers useful are infrared lasers (about 830 nm or 1064 nm), FD-YAG laser (about 532 nm), violet laser (about 405 nm), and ultraviolet laser (such as about 375 nm). Violet and ultraviolet laser sensitive plates are quite attractive because of their yellow light handling capability (in contrast to red light handling for FD-YAG sensitive plate) and higher quantum efficiency (than infrared laser sensitive plate). However, the energy output of a violet or ultraviolet laser diode is much lower than that of an infrared laser diode. In order to be imaged with a violet or ultraviolet laser imager at a practical speed, a violet or ultraviolet laser sensitive plate should have a sensitivity of less than 1000 μJ/cm2, preferably less than 300 μJ/cm2, more preferably less than 200 μJ/cm2, and most preferably less than 100 μJ/cm2, which is significantly lower than the sensitivity required for an infrared laser sensitive plate (typically 50 to 300 μJ/cm2) or a conventional plate exposed with an ultraviolet lamp through a photomask (typically 50 to 300 μJ/cm2).
Silver halide based violet laser sensitive plate has, recently been introduced as the first lithographic printing plate suitable for imaging with violet laser, utilizing the inherently high sensitivity of silver halide. Examples include U.S. Pat. No. 6,541,176. However silver halide based plates have the disadvantages of relatively low run length and generating hazardous silver waste.
Photopolymerizable composition based (also called photopolymer) violet or ultraviolet laser sensitive plates are very attractive because of the high durability of photopolymer plates. However, because of the extremely high sensitivity requirement of the violet or ultraviolet laser sensitive plate, a commercially viable violet or ultraviolet laser sensitive photopolymer plate can not be achieved by simple addition of a laser absorbing sensitizer into a conventional photopolymer plate but will require certain specific photopolymerizable composition. The selection of the monomer, polymer, and intiator is crucial in achieving a commercially viable violet or ultraviolet laser sensitive photopolymer plate.
Violet or ultraviolet laser sensitive plates having a (meth)acrylate monomer, a hexaarylbiimidazole compound as initiator, and a sensitizing dye have been disclosed in the literature.
U.S. Pat. No. 6,689,537 (Urano et al) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises multifunctional (with 3 or more functional groups) urethane acrylate monomers, difunctional non-urethane acrylate monomers, a hexaarylbiimidazole compound, and a sensitizing dye.
U.S. Pat. App. Pub. No. 2003/0186165 (Gries et al) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises difunctional urethane (meth)acrylate monomers, a hexaarylbiimidazole compound, and a sensitizing dye.
U.S. Pat. App. Pub. No. 2006/0024614 (Williamson) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises a difunctional urethane (meth)acrylate monomer, a hexaarylbiimidazole compound, and a sensitizing dye.
U.S. Pat. App. Pub. No. 2005/0266349 (Van Damme et al) describes some specific compositions for violet photopolymer plate developable with a gum solution, wherein the photosensitive layer comprises difunctional urethane (meth)acrylate monomers, a hexaarylbiimidazole compound, and a sensitizing dye.
U.S. Pat. App. Pub. No. 2006/0216646 (Goto et al) describes some specific compositions for violet photopolymer plate, wherein the photosensitive layer comprises a multifunctional non-urethane (meth)acrylate monomer, a hexaarylbiimidazole compound, and a sensitizing dye.
Among the above patents and applications, no specific photosensitive composition containing a multifunctional urethane (meth)acrylate monomer, a multifunctional non-urethane (meth)acrylate monomer, a hexaarylbiimidazole compound, and a sensitizing dye was disclosed.
The inventor has found, surprisingly, violet or ultraviolet laser sensitive lithographic plates having a multifunctional urethane (meth)acrylate monomer, a multifunctional non-urethane (meth)acrylate monomer, a hexaarylbiimidazole compound, and a sensitizing dye can give excellent photospeed and press durability.